Yeah I'm autistic (high functioning) I think I relate to what you're describing. In my own head I always described it as some kind of "lethargy" that pervaded my everyday life. When I tried to describe this to therapists they had no idea what I was talking about. :(
If I'm understanding you correctly, it's the mental side of doing things that takes a long time, not walking or motor skills, etc. It's the same for me too.
Anyway, I feel like at least in my case this "lethargy" is caused by two smaller problems:
1) Daydreaming and gazing off into space too easily and too often. This can happen almost anywhere, but it usually happens at home or anywhere I'm comfortable, such as the shower, bathroom, bedroom, kitchen, etc. I will be in the middle of doing something, then a thought takes me somewhere, then anywhere from 10-30 minutes have passed and I have been standing/sitting doing absolutely nothing, just staring into space. It makes getting up in the morning, getting ready for bed, or going anywhere take FOREVER.
2) Getting confused and lost in details while trying to do things, unsure of what I need to do or the order I should do them in. This can hit me in a lot of places, but it hits the worst whenever there's a task where things need to be done in a specific order. A prime example is cooking, which is why I avoid it. I get jumbled so easily.
With the first issue, I'm honestly still trying to find some kind of solution. I've been trying to notice whenever I get into this daydreamy state, then gently remind myself that I don't need to worry about it or plan/think obsessively when I'm in the middle of trying to do something else. I even allow myself to make a note in my phone to think about it later so that part of me doesn't get upset at having its train of thought shut off. Sometimes I revisit the train of thought if it's really interesting but sometimes I don't.
With the second issue I found that planning my day helps. Both in the morning and the evening I plan what the day (or next day) will have for me. This gets my brain primed for those things and for whatever reason makes doing it more easy. Sometimes, I even do this in the middle of the day for what I have to do the rest of the day. Another thing I found that helps me is just trying to have a clear mind in the moments where I need it. (I know that sounds dumb, let me try to explain.) I found that a lot of the times when I'm feeling jumbled, it's because I'm not able to be 100% with whatever I'm trying to do. My mind is thinking 40% about an unrelated thing and then another 30% about a different unrelated thing, so my brain is trying to make do with the remaining 30%. A lot of the time this can feel like nothing more than an uncomfortable "nagging" when I'm trying to get something done. I usually have to "deal" with those unrelated things in some way to clear them out of my head, resolve them or compartmentalize them. Then I'm able to do what I'm trying to do better. The problem is "dealing" with the "nagging" can take time (sometimes more than 10 minutes) so in reality I can't always do it.
Another thing that came to mind as I was writing this is that if you have OCD, that definitely comes into play. If you ask anyone who knows someone with bad OCD, they will say that people with OCD have a hard time getting out the door to go anywhere. They also have problems with everyday things because they feel like everything has to be done in a very specific way. I have some OCD symptoms and it also contributes to me being slow about so many things.
